1. Field of the Invention
The present invention relates to electrostatic chucks.
2. Related Art Statement
Electrostatic chucks are usually provided with a number of projections or embossed portions projecting from installing faces of their insulating layers and having top faces (contacting faces) to be contacted with semiconductor wafers. Applying DC voltage to an internal electrode inside the insulating layer produces a Johnsen-Rahbek force at the contact interface between the semiconductor wafer and the contact faces of the projections, which attracts the semiconductor wafer onto the contact faces.
At present, high-density plasma is formed above the semiconductor wafer in the case of high-density plasma (HDP) CVD and etching for the semiconductor wafers. In etching, the semiconductor wafer is attracted by means of the electrostatic chuck, and a cooling member is provided under the electrostatic chuck. A rise in the temperature of the semiconductor wafer is prevented by the escaping heat flowing from the high-density plasma to the semiconductor into a side of the electrostatic chuck. In the case of the HDP CVD, the temperature of the semiconductor wafer is controlled to a desired temperature by escaping the heat flowing from the high-density plasma to the semiconductor wafer into the side of the electrostatic chuck from the semiconductor wafer at a constant rate.
In an electrostatic chuck utilizing the Johnsen-Rahbek force, the substrate is made of a semiconductor, and electrons or positive holes are moved inside the substrate. For example, when the substrate is made of an aluminum nitride-based ceramic material, the ceramic material is an n-type semiconductor. The conducting mechanism in the n-type semiconductor relies mainly upon the movement of electrons, and almost no positive holes move.
The present inventors produced electrostatic chucks of the Johnsen-Rahbek type. During this production, it happened that after the electrostatic chucks were joined to the cooling members, the attracting forces for the wafers decreased. In particular, such a problem hardly occurred at voltages of around 100 V, whereas the attracting voltage did not reach the intended level in the case of high voltages not less than 500 V.